7. The facts about serpentine superphosphate
1. Serpentine is a natural magnesium-calcium silicate. It is widely used as a slow release magnesium (Mg) fertiliser in finely ground form. In the case of the product called ‘serp-super’, about 25% by weight of finely ground serpentine is added to the freshly-made superphosphate, before the sulphuric acid has all been utilised in reacting with the phosphate rock to produce soluble phosphate. This means that some of the Mg in the serpentine (maybe 20%) is converted to water soluble form. This may be a slight advantage in the short term, if soil Mg levels are particularly low.
2. The big disadvantage of doing this however, is that the more acid utilised attacking the serpentine rock, the more manufacturing-grade phosphate rock, which is essentially useless agronomically, is remains in the final product. We saw in Quinfacts 6 how even in ordinary super, up to 15% of the total P can be present as unreacted manufacturing rock. In serp-super this can be 25%.
3. So a serp-super advertised as having say 6.8% P, 8% S and 5% Mg, may in fact have only 5% usable P. No mention of this is made in information provided by the superphosphate manufacturers. In my view, this must constitute misleading advertising. The manufacturers cannot have it both ways. They cannot criticise reactive phosphate rock (RPR), which typically dissolves in the soil up to ten times faster than does a manufacturing phosphate rock like Boucraa (from the disputed area adjoining Morocco), yet very clearly imply that all the manufacturing-grade phosphate still present in the super you buy, and even more in serp-super, is plant-available. But this is exactly what they do.
4. Finally, the question of asbestos. Asbestos is actually one of the many crystalline forms of serpentine that can exist in a deposit. Some serpentine in some serpentine deposits is present as asbestos; some fortunately contain none. When a serpentine deposit containing asbestos is mined and crushed, much of the deadly-to-lungs fine asbestos can be released. Much serpentine used to make serp-super in the past was made with asbestos-containing serpentine. More of the dust got released during spreading. It is now illegal to mine serpentine containing more than trace amounts of asbestos.